Winter tires (studless tires) are now being used as pneumatic tires for driving on ice and snow roads in many cases instead of conventional spike tires, and further improvement of performance on ice is much more required. For improved performance on ice and snow, the glass transition temperature (Tg) is preferably lowered so that the elastic modulus at low temperatures (the term “low temperatures” herein means temperatures upon driving on ice and snow, and it is in the range of about −20° C. to 0° C.) is set to be low. Thus, for example, high cis-1,4-polybutadiene is widely used because it has a high cis content and a low Tg and, if it is used for tread rubber of tires, it provides excellent properties such as abrasion resistance and fatigue properties.
In general, however, the elastic modulus at high temperatures tends to be lower as the elastic modulus at low temperatures is reduced; thus, disadvantageously, conventional winter tires are poor in handling stability upon driving on dry roads.
On the other hand, low cis-1,4-polybutadiene, which is mainly obtained by living anion polymerization using an alkyllithium catalyst, improves performance on ice especially in tread rubber containing silica, for example, because the molecular weight thereof is easily controlled and the terminals thereof are relatively easily modified; however, it has a disadvantage in abrasion resistance.
In order to solve the aforementioned problem of reduction in elastic modulus at high temperatures, an experiment has been performed in which, in a silica-containing rubber composition, a specific polar group is attached to the rubber to provide an affinity for silica and to increase dispersibility of silica, so that the reduction in elastic modulus at high temperatures is suppressed. For example, Patent Document 1 discloses an experiment in which rubber is modified by an organosilicon compound containing an amino group and alkoxy group so as to increase the affinity for silica, but the effect is insufficient.    Patent Document 1: JP 2000-344955 A